The present invention relates to a variable displacement pump and, more particularly, to a variable displacement vane pump used in various types of pressure fluid utilizing equipments such as a power steering device for decreasing the force required to operate the steering wheel of a vehicle.
As a pump for a power steering device, generally, a displacement vane pump directly driven to rotate by a vehicle engine is used. In this displacement pump, the discharge flow rate increases or decreases in accordance with the rotation speed of the engine. A power steering device requires a steering auxiliary force which increases while the vehicle is stopped or is maneuvering at a low speed and decreases while the vehicle is maneuvering at a high speed. The characteristics of the displacement pump are contradictory to this steering auxiliary force. Accordingly, a displacement pump having a large volume must be used so that it can maintain a discharge flow rate necessary to produce a required steering auxiliary force even during low-speed driving with a low rotation speed. For high-speed driving with a high rotation speed, a flow control valve that controls the discharge flow rate to a predetermined value or less is indispensable. For these reasons, in the displacement pump, the number of constituent components increases, and the structure and path arrangement are complicated, inevitably leading to an increase in entire size and cost.
In order to solve these inconveniences of the displacement pump, various types of variable displacement vane pumps capable of decreasing the discharge flow rate per revolution (cc/rev) in proportion to an increase in rotation speed are proposed by, e.g., Japanese Patent Laid-Open Nos. 56-143383, 58-93978, and the like. According to these variable displacement pumps, a flow control valve is unnecessary unlike in a displacement pump. Waste of driving power is prevented to provide an excellent energy efficiency. No return flow to the tank occurs to reduce any oil temperature increase. In addition, a leakage in the pump and accordingly a decrease in volumetric efficiency can be prevented.
For example, in the variable displacement pump shown in Japanese Patent Laid-Open No. 56-143383, a cam ring is movably placed in a pump casing. A pair of fluid pressure chambers serving as control chambers are formed in the gap between the cam ring and the pump casing. Pressures before and after an orifice formed midway along the discharge path are guided to these fluid pressure chambers. The differential pressure between these pressures is made to directly act on the cam ring to move it against the biasing force of a spring. As a result, the volume of the pump chamber is changed to perform appropriate discharge flow rate control.
An example of such a variable displacement vane pump will be described briefly with reference to FIG. 12. Referring to FIG. 12, reference numeral 1 denotes a pump body; 1a, an adapter ring; and 2, a cam ring. The cam ring 2 is free to swing and be displaced in an elliptic space 1b, formed in the adapter ring 1a of the pump body 1, through a support shaft 2a. A press means applies a biasing force to the cam ring 2 in a direction indicated by an open arrow F in FIG. 12. A rotor 3 is accommodated in the cam ring 2 to be eccentric on one side to form a pump chamber 4 on the other side. When the rotor 3 is rotationally driven by an external drive source, vanes 3a held to be movable forward/backward in the radial direction are projected and retracted.
In FIG. 12, reference numeral 3b denotes a drive shaft of the rotor 3. The rotor 3 is driven by the drive shaft 3b to rotate in a direction indicated by an arrow in FIG. 12.
A pair of fluid pressure chambers 5 and 6 are formed on two sides around the cam ring 2 in the elliptic space 1b of the adapter ring 1a of the pump body 1, and serve as high- and low-pressure chambers, respectively. Paths 5a and 6a open to the chambers 5 and 6, respectively, to guide the control pressure for swingably displacing the cam ring 2, e.g., the fluid pressures before and after the variable orifice formed in the pump discharge path. When the fluid pressures before and after the variable orifice of the pump discharge path are introduced through the paths 5a and 6a, the cam ring 2 is swingably displaced in a required direction to change the volume in the pump chamber 4, thereby variably controlling the discharge flow rate in accordance with the flow rate on the pump discharge or outlet side. In other words, the discharge flow rate is controlled to decrease with increasing revolution count of the pump.
A pump suction opening 7 is formed to oppose a pump suction region 4A of the pump chamber 4. A pump discharge opening 8 is formed to oppose a pump discharge opening 4B of the pump chamber 4. These openings 7 and 8 are formed in corresponding ones of a pressure plate and a side plate (not shown) serving as stationary wall portions for holding a pump constituent element consisting of the rotor 3 and cam ring 2 by sandwiching it from two sides.
A biasing force is applied to the cam ring 2 from the fluid pressure chamber 6 as indicated by the arrow F in FIG. 12 to maintain the volume in the pump chamber 4 normally at the maximum value. A seal member 2b is placed in the outer peripheral portion of the cam ring 2 to define the fluid pressure chambers 5 and 6, together with the support shaft 2a, on the right and left sides.
Notches 7a and 8a are formed by notching into a substantially V-letter shape, to be continuous to the starting end portions of the pump suction opening 7 and pump discharge opening 8, respectively, in the rotating direction of the rotor 3. Along with rotation of the rotor 3, when the distal ends of the respective vanes 3a are brought into slidable contact with the inner circumferential surface of the cam ring 2 to operate the pump, the notches 7a and 8a having the substantially V-letter shape let the fluid pressure to gradually escape from the high-pressure side to the low-pressure side between a space sandwiched by the vanes close to the end portions of the openings 7 and 8 and a space sandwiched by the vanes adjacent to them. As a result, a surge pressure or resulting pulsation are prevented.
In the variable displacement pump having the above arrangement, a relief valve is formed in part of the pump discharge side to relieve its excessively large fluid pressure.
According to the conventional variable displacement vane pump described above, in a pump cartridge (pump acting portion) formed of the pump constituent element such as the rotor 3 and cam ring 2, pump chambers (chambers partitioned by the vanes 3a; pressure chambers) located at intermediate regions (portions denoted by reference numerals 9A and 9B in FIG. 12) corresponding to a region extending from the end point of the suction opening 7 in the pump chamber 4 to the start point (distal end portion of the notch 8a) of the discharge opening 8 and a region extending from the end point of the discharge opening 8 to the start point (distal end portion of the notch 7a) of the suction opening 7, respectively, change alternately between the pump discharge pressure and the pump suction or intake pressure.
This is due to the following reason. When a vane 3a leading in the rotating direction of the rotor 3 reaches the opening 8 or 7 (notch 8a or 7a) at the distal end side in the rotating direction, the corresponding pump chamber is set at the pump discharge- or suction-side port pressure of this opening 8 or 7 (notch 8a or 7a). When a following vane 3a is at the trailing end-side opening 7 or 8 in the rotating direction, the corresponding pump chamber is set at the port pressure of the following opening.
A thrust generated by the difference between the pressure chambers of the opposing intermediate regions 9A and 9B due to such pressure fluctuation or pressure non-equilibrium acts on the inner surface of the cam ring 2 to vibrate the cam ring 2. As a result, a fluctuation in flow rate or hydraulic pulsation phenomenon occurs on the pump discharge side to cause noise. This pulsation phenomenon appears as shown in, e.g., the graph of FIG. 7(B).
For these reasons, as the variable displacement pump described above, one having an odd number of vanes to relax any pressure fluctuation and pressure non-equilibrium and one in which a variable metering orifice is formed midway along the pump discharge path are proposed. In the latter one, a spool type control valve is switched by the fluid pressures before and after the orifice, and the fluid pressures before and after the orifice and the pump suction pressure are selectively supplied to the chambers 5 and 6 on the two sides of the outer circumferential portion of the cam ring 2, so that vibration of the cam ring 2 is suppressed. However, these countermeasures are insufficient, and a further countermeasure is sought for.
In particular, when the variable displacement pump is used as a hydraulic source for a power steering device that assists the force required to operate the steering wheel of a vehicle, the pump is rotationally driven in accordance with rotation of the engine. In this pump, the cam ring 2 swings by fluctuation in rotation speed of the engine, and its position with respect to the pump chamber 4 changes. When the cam ring 2 swings and is displaced in this manner, the positions of the pump suction opening 7 and pump discharge opening 8 as port grooves formed in the pump chamber 4 and the position of the cam ring 2 relative to each other change.
When swingable displacement of the cam ring 2 changes the position of the cam ring 2 relative to the port grooves described above in this manner, the timing at which the notches 7a and 8a, formed by notching in the starting end portions of the openings 7 and 8 in the rotor rotating direction to have a substantially V-letter shape, communicate with the pressure chambers (pump chambers) between the vanes 3a changes. This is due to the following reason. In the conventional pump, the notches 7a and 8a described above are formed to have fixed positions in the radial direction with respect to the pump chamber 4 and fixed lengths in the rotating direction. Hence, while the engine idles, the notches 7a and 8b communicate with the pressure chambers to allow smooth pressure fluctuation.
More specifically, with the notches 7a and 8b formed at the positions as described above, the pressure change occurring when the pressure chambers communicate with the corresponding openings 7 and 8 is relaxed, and the noise accompanying sharp pressure fluctuation can be decreased. If, however, this communication timing is set to match idling, when the rotation speed is high, the notches 7a and 8b do not function, and the noise increases.